This screenshot shows a history based suggestion. The mouse cursor was hovering over the first line and that made a tooltip appear with the complete text of the suggestion. The tooltip only appears for those items where the text of the suggestion is too large to fit into the popup.
Hovering requires abandoning the keyboard in favor of the mouse to move the cursor there and wait for the tooltip trigger timeout. It could be much quicker if that tooltip would appear not only when hovering the cursor over it but immediately when using up and down arrows to move the current selection.